Bus Control Pins - Motorola MC68302 User Manual

5.6 DATA
BUS PINS
015-DO
This 16-bit, bidirectional, three-state bus is the general-purpose data path. It
can transmit and accept data in either word or byte lengths. For all 16-bit
IMP accesses, byte 0, the high-order byte of a word, is available on 015-08,
conforming to the standard M68000 format.
When working with an 8-bit bus (BUSW
=
O), the data is transferred through
the low-order byte (7-0). The high-order byte is not used.
5. 7 BUS CONTROL PINS
5-6
AS - Address Strobe
This bidirectional signal indicates that there is a valid address on the ad-
dress bus. This line is an output when the IMP (M68000 core, SOMA or
IDMA) is the bus master, and is an input otherwise.
R/W - Read/Write
This bidirectional signal defines the data bus transfer as a read or write
cycle. It is an output when the IMP is the bus master and is an input
otherwise.
UDS/AO - Upper Data Strobe/Address 0
This bidirectional line controls the flow of data on the data bus. When using
a 16-bit data bus, this pin functions as upper data strobe (UDS). When
using an 8-bit data bus, this pin functions as AO.
This line is an output when the IMP is the bus master, and is an input
otherwise.
LDS/DS - Lower Data Strobe/Data Strobe
This bidirectional line controls the flow of data on the data bus. When using
a 16-bit data bus, this pin functions as lower data strobe (LDS). When using
an 8-bit data bus, this pin functions as DS. This line is an output when the
IMP (M68000 core, SOMA or IDMA) is the bus master and is an input
otherwise.
DTACK- Data Transfer Acknowledge.
This bidirectional signal indicates that the data transfer has been com-
pleted. DTACK can be generated internally in the chip-select logic either
for an IMP bus master or for an external bus master access to an external
address within the chip-select ranges. It will also be generated internally
during any access to the on-chip dual-port RAM or internal registers. If
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